The visual system is continuously recalibrated through processes of adaptation that adjust sensitivity in response to changes in the environment or the observer. These adaptive adjustments operate throughout the visual system and thus profoundly influence most if not all aspects of visual perception. Most prior studies of visual adaptation have focused on sensitivity changes over short times scales ranging from milliseconds to minutes. However, there are increasing theoretical arguments and empirical evidence that these rapid adjustments are superimposed on response changes that influence visual coding over much longer durations. These long-term forms of adaptation may be the primary mechanisms for setting the baseline operating states of the visual system, and may be especially important for compensating visual coding for the gradual but pronounced changes in optical and neural properties of the visual system that occur throughout the lifespan. Yet almost nothing is known about the nature or properties of adaptation at longer timescales, or about which attributes of the visual stimulus they can adjust to. The proposed research will provide a systematic investigation of long-term visual adaptation through a series of studies that will probe the mechanisms and sites of long-term sensitivity adjustments and their consequences for visual perception and visual performance. Specific Aim 1 will probe the effects of adaptation to changes in the average color of the environment or in the spectral sensitivity of the observer, and how these effects arise from sensitivity changes at both retinal and cortical sites. Specific Aim 2 will test whether the long-term response changes demonstrated for color are also manifest for other stimulus attributes, including the cortical mechanisms mediating contrast and blur perception as well as high-level aftereffects for face recognition. These experiments will help to reveal whether long-term adaptation plays a general or specialized role in calibrating visual responses. Aim 3 will test whether the functional form of the response changes differs at short and long timescales, and thus whether they are designed to adapt visual coding for different aspects of the stimulus. Finally, Aim 4 will explore the consequences of long-term adaptation for optimizing visual coding. These experiments will include altering the color characteristics of images to simulate how they should appear under theoretically complete adaptation, and then testing how well observers can respond to information in these pre-adapted images. The results of these studies will help to reveal the mechanisms, properties, and consequences of adaptation processes that are likely to play a central role in normal visual function but which remain largely unexplored.